The present invention is concerned with the power economy of a crushing plant in any situation where the product of the plant has an economic value that increases with decreasing particle size. As a typical example from which the utility and importance of the present invention will become readily apparent and which illustrates the complex problem that the invention solves, a crushing plant for mine-run iron ore may be operated in conjunction with a grinding mill to which ore must be fed in the form of particles that are below a specified size. Typically, the material fed to the grinding mill should be capable of passing a 1/2 inch (12.7 mm.) mesh screen. The mine-run material is processed through the crushing plant before being fed to the grinding mill in order to reduce the larger chunks and particles of the mine-run ore to grinding mill feed size.
It is well known that a crushing plant utilizes power more efficiently than a grinding mill. Thus, other things being equal, a crushing plant needs about half as much power as a grinding mill to reduce particle size by a given amount. Of course a crushing plant cannot reduce material to the very small and uniform particle size for which the grinding mill is needed, and therefore it is not possible to eliminate the grinding mill. But a crushing plant can turn out product in a range of particle sizes that can be fed to a grinding mill, and to the extent that product which is at or near the lower end of that range can be obtained from the crushing plant, the comminution work that is done by the more efficient crushing plant need not be done by the less efficient grinding mill, so that there is a net saving in the power needed to reduce the material to its ultimate particle size.
Heretofore it has not been known how to take advantage of the high comminuting efficiency of a crushing plant in order to obtain an economically optimum product. In fact, the operator of such a plant, who is usually assigned a quota of finished product for each working day or similar working period, often acted under the belief that he was achieving the greatest efficiency when he completed his quota within the shortest possible time and could thus conserve energy by shutting down the plant well before the end of the working day. Indeed, this theory of crushing plant economy has become so well established and so widely accepted that considerable ingenuity has been devoted to the provision of methods and apparatus for maximizing crushing plant tonnage per unit of time. See, for example, U.S. Pat. No. 3,480,212 to Liljegren et al, which discloses apparatus that is designed (according to the "Summary of Invention") to keep a crushing plant "operating at maximum tonnage by automatically checking certain operating conditions and thereafter automatically adjusting the set point of the automatic controller in the proper direction to obtain maximum feed rate for the existing conditions". Again, U.S. Pat. No. 3,078,051, to Patterson, discloses an automatically controlled crusher which, the patent says "produces a substantially constant tonnage per hour for a given horsepower consumed by the crusher. In this way the crusher produces a maximum tonnage output for the power consumed by the machine and hence operates at or near its peak efficiency for the material being crushed".
The present invention is based upon a recognition that there was a very serious fallacy in the reasoning whereby maximizing tonnage was set as the goal for crushing plant operation, in that such reasoning failed to take account of the economic value of the product of the crushing plant and therefore led to production of low value product. The more rational premise of the present invention is that a crushing plant is operated most profitably and most efficiently from the standpoint of both power consumption and capital utilization when its product is turned out at such a rate as to rather accurately meet a daily quota which is reasonable for the power available to the plant, and when, furthermore, the product has the highest economic value attainable within the constraints of the quota and the available power.
The invention further proceeds upon a recognition that the value of the product of a crushing plant is more or less directly related to the amount of energy that is expended by the plant in crushing a given quantity of the product, owing to a relationship between power expended and product particle size that is explained hereinafter.
These premises of the present invention are perhaps not new ideas in themselves, and their significance may have been appreciated in the past, but heretofore, considered in relation to one another, they have posed a baffling dilemma for the operator of a crushing plant. If he operated the plant in such a manner as to obtain a product of maximum economic value, he was likely to fall short of his production quota; and if he operated with his quota in mind, he could only follow the prior art teachings that set maximum tons per hour as the goal. The problem was aggravated by certain factors that greatly complicate the problem of controlling a crushing plant to achieve both quota fulfillment and optimum product value.
One of these complicating factors is the variable crushability of the material to be fed into the plant. Some pieces of material are more easily crushed than others, and a run of easily crushed material reduces the power required for crushing, or speeds up the throughput of the plant, or both.
Another complicating factor is the wide variation in size of the input material particles. For material of a given crushability, power required for crushing is a function of reduction ratio which is the ratio of the size of uncrushed particles to crushed particles. Other factors also bear upon the power required, but, in general, less power is required to crush small particles to a given final size than to crush large ones to the same final size. Therefore, assuming a constant power application and that both large and small particles are crushed to the same final size, a quantity of raw material consisting mostly of small particles can be crushed more rapidly than one containing mostly large particles.
Another complicating factor is that the mine-run infeed material enters the crushing plant at a separating zone where the smallest particles are separated from the remainder of the material and from which they are transferred directly to a delivery zone in bypassing relation to the crushing mechanism. Since the separated fine material must be considered as a part of the production of the plant that contributes to fulfilling its quota, optimizing production requires that the full available power of the plant be applied to the larger size remainder of the material and that exactly so much of that material is put through the plant during the day as will, together with the unpredictable volume of fines that have bypassed the crushing mechanism, make up the day's quota.
There are other complicating factors, some of which may be unknown, inasmuch as no mathematical model has been found that accurately states the relationship between rate of production and power required at any given time. Nevertheless, in the operation of a crushing plant in accordance with the method of this invention, that varying and unpredictable relationship is constantly taken into account in a very simple manner.
From what has been said above, it will be apparent that the general object of this invention is to provide a method of so controlling operation of a crushing plant that the output of the plant will consistently be substantially equal to a daily quota established on the basis of an assessment of the reasonable capabilities of the plant and, in addition, that its product will have the optimum economic value attainable with the expenditure of all of the power available to the plant, having in mind that the economic value of a given quantity of crushing plant product increases with increase in the power expended to produce that quantity of product.
In the most general terms, the object of the present invention is conservation of energy, as will be apparent when the invention is considered in relation to a crushing plant which feeds into a grinding mill, in which case the invention has as its object the processing of any given amount of material through the entire complex comprising the plant and the mill with a minimum expenditure of energy for the total processing.
Still speaking very generally, it is also an important object of this invention to provide a method of so controlling the operation of a mineral crushing plant as to achieve optimum utilization of the capital invested in the plant.
Another and more specific object of this invention is to provide a method for so controlling the operation of a crushing plant that an assigned production quota will be met by it at the end of each working day or similar working period, notwithstanding constant variation in crushability and size of the raw material fed into the plant and the varying rates at which product-size fines are bypassed around the crushing mechanism.
It is also a specific object of this invention to provide a method and process whereby a crushing plant may be controlled to achieve the several objects set forth above, either with the employment of manual controls, or with fully automatic controls that can be relatively simple and inexpensive, or with a combination of manual and automatic controls.
Inasmuch as recirculation of material in a crushing plant is inefficient in consuming power for mere transportation of material within the plant and in requiring the presence of expensive screening and classifying equipment for the recirculated loads, it is another specific object of this invention, realized in certain modes thereof, to provide a method of so operating a crushing plant as to minimize or avoid recirculation of materials while at the same time attaining the objectives set forth above.